Photogrammetry
Ground sampling distance, overlap, image count, blur, and accuracy.
The numbers behind a mapping flight: ground sample distance, the height for a target GSD, overlap, trigger interval and line spacing, image count, motion blur, overlap over high ground, oblique footprints, and accuracy to the ASPRS Edition 2 standard. It is for drone mapping crews and surveyors planning photogrammetry. Start with the GSD tool, then overlap and trigger. Use the camera's physical focal length, not the 35 mm equivalent on the spec sheet, or the GSD comes out far too fine.
Planning and education aid. Not for primary navigation. Full disclaimer
I want to find the GSD for a height, or the height for a GSD
- Ground sampling distance (GSD)
How much ground each pixel covers, and the image footprint, from the camera and height above ground; catches 35 mm-equivalent focal lengths entered by mistake.
- Height for a target GSD
The height above ground that gives a target ground sampling distance with your camera, checked against your altitude ceiling.
I want to find the GSD and footprint of a tilted (oblique) camera
- Oblique GSD and footprint
For a camera tilted off straight down, the ground each pixel covers at the image center and its near and far edges, the trapezoid the image covers, and a warning when the view reaches the horizon.
I want to check overlap where the ground rises
- Overlap over high terrain
A mission flown at one height above takeoff is closer to the ground over hills, so photos cover less and overlap drops. The lowest overlap and the height left over the highest ground.
I want to set overlap, trigger interval, and line spacing
- Overlap, trigger interval, and line spacing
Photo spacing, trigger interval, and flight-line spacing for your front and side overlap, with a check that the camera can keep up.
I want to estimate how many images a job takes
- Image count and survey size
How many photos, flight lines, and kilometers a mapping survey of an area takes at your line and photo spacing, counted the way published flight planning counts them and from the same sweep the survey-grid tool flies.
I want to check for motion blur
- Motion blur and slowest shutter
How many pixels the image smears at your speed and shutter, and the slowest shutter that keeps blur under a limit.
I want to state accuracy to the ASPRS standard
- ASPRS accuracy (Edition 2)
Horizontal and vertical accuracy by the ASPRS Positional Accuracy Standards, Edition 2, including the checkpoint survey's own error and the 30-checkpoint minimum.
Planning a mapping flight
- Pick the ground sampling distance the deliverable needs, and find the height that gives it with your camera. Height for a target GSD
- Choose front and side overlap; they set the photo interval and the spacing between lines. Overlap, trigger interval, and line spacing
- Check the shutter speed keeps motion blur under a fraction of a pixel at your groundspeed. Motion blur and slowest shutter
- Estimate the image count for the area, which drives batteries and processing time. Image count and survey size
- After processing, report accuracy against checkpoints to the ASPRS standard. ASPRS accuracy (Edition 2)
Guides that use these tools
- Drone mapping day: From the resolution the client wants to the height, overlap, blur check, image count, and the hours of good light.
- Lidar bid: The size of the site, the flight lines and flight time, the batteries it takes, and the accuracy you can state.